Another marine dipterous insect is a gnat-like fly closely allied to _Chironomus_, which we have described in a former work[A] of this series dealing with fresh-water life; and it will be sufficient to mention here that _Chironomus_ is commonly known as the window-gnat on account of the frequency with which it may be seen flying on the windows of our dwellings; also that the larva, known popularly as the bloodworm, is truly aquatic in habit, being able to swim by rapidly looping its body in opposite directions, and being provided with a breathing apparatus adapted for the absorption of the oxygen gas contained in solution in water. The larva of the marine species referred to above may sometimes be seen in rock pools, where it shelters itself among the sediment at the bottom. It is much like the bloodworm in appearance and structure, but its body is greenish instead of red. The last order of insects calling for notice here is the _Coleoptera_ or sheath-winged insects, popularly known as beetles, and characterised by the hard and horny nature of the front pair of wings (elytra), which are modified into sheaths and serve to protect the second pair; the latter are thin and membranous, usually adapted for flight, and lie folded beneath the former when not in use. One large section of beetles is known as the _Geodephaga_ or Ground Beetles--a group of very predaceous insects that burrow into the soil and attack almost every living thing that comes in their way, and well represented by numerous species that may be found in our gardens, and, in fact, almost everywhere. A considerable number of these insects show a decided preference for salt marshes and the sea shore, where they hide under stones, or burrow into the sand or mud in search of their prey. They are not marine in the strictest sense of the word, for they are not adapted for a life of submersion in water, either in the larval or in the perfect condition; yet they are often found below high-water level, and some species burrow into the sand of the beach as the tide advances, allowing the water to cover them for hours together. One interesting family of the ground beetles (the _Bembidiidæ_) includes several small species, all of which frequent salt and wet places, such as salt marshes, the mouths of rivers, and the sea shore. We give enlarged illustrations of a few of these, the actual size being denoted at the side of each. [Footnote A: _Life in Ponds and Streams._] [Illustration: FIG. 223.--MARINE BEETLES OF THE GENUS _Bembidium_ 1. _B. biguttatum._ 2. _B. pallidipenne._ 3. _B. fumigatum._ 4. _B. quadriguttatum_]